Interpretive Summary: Loss of offspring during pregnancy represents a major inefficiency in cattle, sheep, and pigs. Mechanisms that contribute to these losses share some commonalities, but others are specific to the different livestock species. Two common factors that influence the outcome of pregnancy in all three species are the migration and elongation of embryos during early pregnancy. Livestock embryos are capable of moving within the uterus, which helps them avoid each other when multiple offspring are present. They also transform from a spherical shape to a threadlike shape within the uterus, which allows the developing embryo to come into contact with more of the inside lining of the uterus, and controls the eventual size of the placenta. The size of the placenta contributes to the effective transport of nutrients to the developing offspring during pregnancy. Although all livestock embryos do this, the embryos of cattle, sheep, and swine differ in their ability to migrate and the consequences of coming into contact with each other during elongation. Swine and sheep embryos migrate efficiently and do not usually interfere with each other if they come into contact. Cattle embryos migrate poorly. In addition, when cattle embryos contact each other, the elongating embryos merge with each other and share blood supplies, which causes several problems during later pregnancy. Another commonality between species is the importance of uterine glands. Uterine glands are structures within the inside lining of the uterus that secrete substances that are essential for the development of the offspring. Improper development of uterine glands reduces fertility. Recent evidence indicates that failure to receive colostrum shortly after birth impairs both uterine gland development and fertility. A final commonality among livestock species is that the placenta converts a substantial portion of the sugar it transports from the mother into fructose, but very little is known regarding the role of fructose during pregnancy in any livestock species. Recent evidence suggests that fructose may be used to synthesize major components of the placenta, suggesting that fructose may promote placental development leading to improved placental efficiency and reduced loss of offspring in livestock.

Technical Abstract:
Conceptus losses in livestock occur throughout gestation. The uterus and the embryo– placenta–fetus play interconnected roles in these losses, the details of which depend on the period of gestation and the species. Studies in sheep and pigs have indicated that the uterine glands are essential for full fertility, based on experiments where gland development was reduced through the use of exogenous hormones. In sheep and cattle, normally the uterus is well able to support more than a single fetus although these species differ in the consequences of multiple births. When 2 conceptuses are present, the placentas of cattle often anastomose, putting 1 fetus at risk if the other is lost. One likely reason this does not occur in sheep is because sheep embryos undergo intrauterine migration, similar to pigs. In pigs, the relatively equidistant separation of conceptuses is likely to be essential for optimizing conceptus survival as is the simultaneous and uniform elongation of blastocysts that occurs during the time of maternal recognition of pregnancy. Other studies in pigs have indicated that the size of the uterus influences litter size and therefore fetal losses. In response to crowded intrauterine conditions in the pig, increased conceptus losses begin to occur between d 30 and 40 of pregnancy, and further losses occur sporadically during later gestation. There is evidence that improved fetal erythropoiesis can reduce these losses. Other studies indicated that profound changes in placental development occurred under crowded intrauterine conditions that may contribute to losses during late gestation. Reductions in placental stroma formation may compromise the ability of the pig placenta to adapt to reduced uterine space. Consistent with this, both hyaluronan and hyaluronidase activity are decreased in the placentas of small compared with large fetuses. These results indicate that improvements in placental stroma formation could improve placental ability to compensate for reduced intrauterine space, resulting in increased placental function and reduced fetal losses during late gestation.